The problem of laser beam stabilisation over long beam paths is not new, but is one which is becoming more important due to the growth of large laser facilities throughout the world. It is also becoming more relevant due to the emergence of greater interest in lasers from the nuclear, automotive, aerospace and ship building industries who are interested in remote processing and multiple work station operations.
In high power lasers, the large thermal load experienced by the cavity resonator leads to distortion of the cavity resonator mirrors and structure, thereby misaligning the cavity and hence inducing changes in spatial mode and beam direction. As the characteristics of most material processing interactions and critically dependent on the optical parameters, of the radiation, the need for stabilisation becomes apparent. In particular, flexible manufacturing may require the laser to be switched on and off, and so change power level, many times during a processing operation. When a long beam path (greater than 20 meters) is involved, the problems are exaggerated and stabilisation of beam pointing, mode and beam diameter becomes highly desirable.
Initial approaches to stabilising the mode and power of high power lasers (eg CO.sub.2 lasers) concentrated on the incorporation of elaborate optical benches into the laser designs. However, even with such an approach, operator intervention was still found to be necessary. More recently, in a series of approaches with increasing complexity, automatic control of beam mode and power of a high power CO.sub.2 laser has been carried out by others. A rotating wand sampler has been used to couple 1 per cent of the laser output onto a quadrant thermister array. By comparing average power in each quadrant, feedback signals were used to realign the rear high reflector using motor driven micrometers. Mode and power were stabilised and the system response time was about 2 minutes. In a development of this system a much faster pyro-electric quadrant sensor array in the form of a ring was introduced to match the intensity profile of the laser output in the near field. Feedback to two mirrors within the cavity structure was employed to maintain beam mode and, in addition, a separate, closed loop was established for controlling output power. This resulted in a greatly improved performance with a response time of about 10 seconds. By replacing the pyro-electric sensor array with a thermal imaging screen, video camera and frame grabber for digitising the image, and using image processing techniques, the uniformity of the spatial mode and beam position could be measured and appropriate control signals applied for optimisation. The resulting mode and power stabilisation produced stabilised pointing stability of around 60 .mu.rad and power stability of less than 4 per cent.
The concept of using partially transmitting or partially reflecting mirrors of different types in laser beam position sensors is known. GB 2184831A describes a mirror having an array of small holes which is used to provide a spatial sample of an incident laser beam, the sampled laser radiation being brought to a focus on a quadrant pyro-electric detector. A disadvantage of this system is that only a small fraction of the incident beam is sampled since the sum of the area of the small holes must be very much less than the area of the laser beam otherwise an unacceptably large portion of the incident beam would be lost through the mirror.
FR 2616555 describes a system in which a laser beam is reflected by a partially-reflecting mirror. The reflected beam is used as a low intensity beam which passes through a further partially reflecting mirror which divides the low intensity beam into two paths, each feeding a quadrant optical sensor. The primary beam is transmitted through the first mentioned mirror. However, it is generally recognised that, when using high power lasers, routing the primary beam from the laser source to a target through a transmitting optical component should be avoided. Thus, the system described in FR 2616555 is unsuitable for use with high power lasers.
Another known system which uses partially-reflecting optical components and quadrant sensors is described in U.S. Pat. No. 4,618,759. In an embodiment of that system, a separate pilot laser beam, parallel to the primary laser beam, is used for detecting beam position. However, this known system suffers from the problem that the misalignment that the primary beam undergoes cannot be compensated for, since the measurement is not carried out on the primary beam.